The 4 types of epidemic, and their characteristics

Epidemics are not new events in the history of humanity, but have occurred throughout it. The black plague, measles or the Spanish flu are examples of this. The appearance of new infectious diseases have significantly influenced the future of human evolution.

The generation of infectious outbreaks is not unusual, it is more common than many may think. But thanks to the advancement of our knowledge in the field of health, the impact is less than it could be in the past. The study of these phenomena has allowed us to know more about them, how they originate or differentiate different types of epidemics, and thus have an advantage when fighting with their consequences.

What is an epidemic?

We cannot continue with the subject if we do not know this concept before. From the Greek Epi, which means "on", and Demos, which is "people", it is understood as an epidemic when there is an increase significantly high number of cases of a specific disease in a specific location, over a period of time determined. As can be extracted from this definition, in order to affirm that an epidemic outbreak is occurring you must have a rapid spread of a particular condition in a given population in a very short time weather.

This term was previously associated with infectious diseases, that is, discomforts that are caused by invasive external agents (bacteria, virus,...); and contagious, that there is transmission between person to person, but as we will see with the different types of epidemics, this word has been extended to other kinds of affections. Even the World Health Organization (WHO) rates the obesity like an epidemic.

How Outbreaks Happen

It is clear that if there is no disease, there can be no epidemic outbreaks. But as we can see they always appear over time, and although currently there is specifically talk of the COVID-19 due to its global impact, which will be discussed later, some type of outbreak appears from time to time in some country.

In the simplest case to explain it is with an infectious disease. The pathogen or infectious agent, for example viruses or parasites, is found in a natural reservoir, such as stagnant water or within a specific animal that does not cause symptoms, and when environmental conditions change, whether it makes a temperature lower or higher environmental humidity, it can be transmitted and reach people, causing discomfort in them, activating their system immune.

As we can see, there are many factors for this to occur, and more for it to become a type of epidemic, such as a increased ability of the pathogen to cause disease (a concept known as virulence), environmental conditions (temperature, humidity ...) and conditions of the host, in our case people (current level of immune system, deterioration due to previous affections, ...).

Classes of epidemics

The scientists in charge of the study of these phenomena, in other words, epidemiologists have looked for any type of information that can provide the appearance of outbreaks for better prevention against them.

For example, how it originates, and this is the criterion used to differentiate the different types of epidemics that exist, which are the following.

1. Of common origin

This category includes the outbreaks that originate when a group of people is infected by exposure to a common source but there is no person-to-person transmission. With an example it is much better understood. In a fictitious case, a group of people from a restaurant are admitted to a hospital due to discomfort in the digestive system due to a Salmonella infection. As we can extract from here, the common origin of all these patients is to have eaten in the aforementioned restaurant food that contained infectious bacteria, but there has been no transmission between the people who were in the premises, since Salmonella does not have that ability.

This type of epidemic is subdivided into three subcategories, the first being that we have exposed the case of "punctual"In other words, it only took a single moment of contact with the source of the infection to obtain the condition.

Another of the subtypes is "continuous", which is when continuous contact with the source of the problem is required to fall into the disease. An example would be if there was a breakdown in the water purification system, and the people of the town that received it drank continuously from it, there would come a time that would have problems in the digestive system due to the bacteria that live in the water, but they need to be drinking continuously over time to get to present symptom.

Lastly, there would be the cases of "intermittent", which would be similar to the previous one, but a continuous exposure to the origin is not necessary, but neither is it contagious with a single contact.

2. Propagation

In this type of epidemic there is no common source but it spreads from person to person, with great frequency and more and more are affected by the disease. A clear example is the seasonal flu caused by the influenza virus.

Transmission can be direct (through the air, through sexual intercourse, ...) but also indirectly such as the use of vehicles (for example, the contagion of HIV virus by sharing syringes) or by vectors (example of yellow fever and the mosquito that bites between people).

3. Mixed

In the next kind of epidemic it would be the case of the mixed one, which is the mixture of the previous two. That is, people get the infection through a common source but then they transmit it to other people. We can put the case of an outbreak of shigellosis, which translates into gastrointestinal infection, which occurs in a population during the celebration of its parties.

People ingest a product in it that carries Shigella, the bacteria that causes the disease. People return to their municipalities of origin after the party and after an incubation period they show symptoms and infect other people around. The source of origin is a product at the party but the spread is due to people who infect others and have taken it to other points far from the origin. The truth is that these cases are the most colorful.

4. Others

And finally, here are grouped the types of epidemics that do not meet the requirements to be considered part of the above. Cases of zoonoses are included here, which are diseases transmitted from animals to humans only.

Example are the Lyme's desease, a condition caused by the bacterium of the genus Borrelia that is transmitted by rodent ticks to humans. Obesity could also be included, since it is a complex disorder but the WHO qualifies it as epidemic, but that is already more complicated to speak, since there is no class of infectious agent or contagion.

Endemic, epidemic and pandemic

Although these are not types of epidemics, they are three concepts that are closely linked to each other. If we remember, an epidemic would be an event that occurs an unusual increase in patients affected by the same condition, in a given population in a specific period of time. But in the event that this disease lasts over time and appears periodically in a specific location, then it will become endemic.

On the other hand, if an epidemic outbreak moves quickly to more locations, reaching a worldwide spread, then we speak of a pandemic. A curious fact about this is that in 2009, the WHO changed its definition, since before, in addition to affecting several countries simultaneously, it also had to present a high mortality to consider an outbreak as pandemic. Now it is not necessary for the disease to have a high lethality to be classified in this way.

Unfortunately, these days it is something that we have been able to verify as a family virus Coronaviridae, SARS-CoV-2 (known as Coronavirus), has been transmitted from animals (the pangolin, an armored mammal, was pointed out) to humans, and has subsequently had a rapid spread in the population of China, being at that first an epidemic outbreak, then going on to a rapid worldwide dispersion, moment that went on to be classified into a pandemic. The problem with this is that it is a new condition, so we must continue to study and follow the recommendations that we receive from the scientific community.

It is in the hands of all of us to follow the advice so as not to get infected and thus reduce the number of infected, to stop the advance of the disease and be able to attend to all patients without saturating the country's health.

Bibliographic references:

• Juan Pablo Horcajada and Belén Padilla. “Endemic and epidemic. Investigation of a nosocomial epidemic outbreak ”. 2012. https://seimc.org/contenidos/documentoscientificos/eimc/seimc_eimc_v31n03p181a186.pdf

• U.S. Deparment of Health and Human Services. "Principles of Epidemiology in Public Health Practice". Third edition. 2012. https://www.cdc.gov/csels/dsepd/ss1978/SS1978.pdf